Inactive substances that are used in the formulation of drugs other than the Active Pharmaceutical Ingredients (API) are defined as excipients. Excipients act as a carrier for active pharmaceutical drugs. The excipients present in the drug help to achieve prolonged stability of the drug and influence a number of factors such as hardness, friability, uniformity of content (UC), coating, speed of disintegration, dissolution, bio-availability enhancement, compressibility, mixing, and fluidity of drugs. API (Active Pharmaceutical Ingredient) works on the drug’s therapeutic effect, and the excipient’s role is to guarantee the overall performance of medication in terms of stability, preservation, coloring, flavouring, lubricating, etc., to optimize the therapeutic effect of the drug. Excipients do not possess any medicinal properties but play a vital role in facilitating the absorption of drugs. Excipients are either used individually or in combination and are selected on the basis of their compatibility with drugs’ API (Active Pharmaceutical ingredients).
Ideal properties of excipients
1. They must not interact with the drug to affect its therapeutic effect
2. Pharmacologically inert and inactive
4. Non-toxic and FDA approved
5. Organoleptic properties
6. Highly efficient
7. Impurities free
Types of excipients
Diluents, also known as fillers or bulking agents, are used to increase the volume or weight of the drug. These are inert substances that can be useful in achieving the desired volume, flow properties, and compressibility for the drug. Diluents are a vital additive for tablets and capsules. Some commonly used diluents are dibasic calcium phosphate, kaolin, lactose, mannitol, microcrystalline cellulose, powdered cellulose, precipitated calcium carbonate, sorbitol, and starch. Choosing a suitable diluent for a drug depends on various factors like cost, interaction with the drug, and compatibility, for example, a calcium-based salt cannot be used in the manufacturing of tetracycline antibiotics as calcium affects the absorption of the antibiotics from the gastrointestinal tract. Diluents can be classified as-
Preservatives are the substances used in the drug manufacturing process to protect drugs from undergoing undesirable chemical changes and microbial spoilage such as bacteria, fungi, and other microorganisms to increase the shelf life of the drug. These are most commonly used in liquid and semi-solid formulations. Anti-fungal preservatives prevent the growth the fungi. Butylparaben, Propylparaben, and Ethylparaben are common examples of anti-fungal preservatives. The preservatives used to inhibit the growth of bacteria are known as anti-bacterial preservatives, for example-Benzoic acid.d
Binders are responsible for the adhesive nature of tablets and other pharmaceutical formulations as it helps the granules to hold together and maintain integrity. Binders add mechanical strength to tablet granules. Carboxymethylcellulose sodium, Ethylcellulose, Gelatin, Acacia gum, Methylcellulose, and Liquid glucose are some of the common pharmaceutical binders.
Disintegrating agents or disintegrants are usually added to oral dosage forms like tablets, which are intended to break down into smaller particles after the administration of the drug for rapid dissolution of the drug in the GI (gastrointestinal tract) fluid. Examples of disintegrants are alginic acid, polacrilin potassium (e.g., Amberlite), sodium alginate, sodium starch glycolate, and starch.
Lubricants and glidants (Flow modifier agents)
Lubricants are used in solid dosage forms (tablets and capsules) to prevent the adhesion of granules to dies and punches during their compression in the granulation process, which promotes the smooth ejection of powder granules. Calcium stearate, magnesium stearate, mineral oil, stearic acid, and zinc stearate are examples of lubricants. Magnesium stearate is the most widely used tableting lubricant. Lubricants can also be used in power blends for capsule filling, where compression is not involved, to prevent adherence of granules to equipment surfaces.
Glidants are the excipients used in solid dosage forms to reduce the friction between the particles/granules, promoting the smooth flow of powder particles. They are also known as anti-caking agents, according to USP (United States Pharmacopoeia), which prevent caking or clumping of granules when stored in bulk. Examples – Talc, corn starch, aerosil, and syloid.
Acidifying and Alkalizing agents
Acidifying agents are used in liquid dosage forms to provide an acidic medium to enhance the stability of the drug. Examples of acidifying agents are citric acid, acetic acid, fumaric acid, hydrochloric acid, and nitric acid.
Alkalizing agents are used in liquid dosage forms to provide an alkaline medium to contribute to product stability. Examples of alkalizing agents are ammonia solution, ammonium carbonate, diethanolamine, monoethanolamine, potassium hydroxide, sodium bicarbonate, sodium borate, sodium carbonate, sodium hydroxide, and trolamine.
It is an ability of an agent to adhere other molecules onto its surface by physical or chemical means. Powdered cellulose and activated charcoal are examples of such agents.
Flavouring agents are chemical substances that are used to add a pleasant taste and even odour to the product. It may be either natural or synthetic. Apart from natural flavouring agents, some synthetic flavouring agents include-
- Anise oil
- Cinnamon oil
- Orange oil
- Peppermint oil
- Mint oil
Fillers are used to increase the volume of the preparation to make its size suitable for consumption. They also help to improve the stability of drugs.
Buffering agents (pH adjusting agents)
Buffering agents are the agents used to resist the change in pH upon dilution or on the addition of an acid or alkali. Some of the buffering agents are –
- Potassium metaphosphate
- Potassium phosphate
- Monobasic Sodium acetate
- Sodium citrate
- Anhydrous and dihydrate
Sweetening agents are chemical substances that are used to mask the unpleasant or bitter taste of partially dissolved drugs in various pharmaceutical dosage forms. It is widely used for oral preparations, especially in chewing tablets. Sweetening agents can also be used for lozenges, oral disintegrating tablets, dispersible tablets, oral solutions, emulsions, and oral suspensions. Sucrose is a highly used sweetening agent. Examples of sweetening agents are –
- Saccharin sodium
Plasticizers (coating agents)
Plasticizers are the components of film-coating solutions added to polymers to make the film pliable and easy to adjust (flexible). These are mainly used for solid-oral dosage forms like tablets. They enhance the spread of coats over tablets, beads, and granules. Diethyl phthalate and Glycerin are common plasticizers.
Surfactants (surface active substances)
Surfactants, as the name suggests, are surface-acting agents, used to alleviate (reduce) the surface or interfacial tension to increase the hygroscopicity of tablets. Surfactants can be used as a wetting agent, detergent, or emulsifier. Examples of surfactants include Benzalkonium chloride, Nonoxynol 10, Octoxynol 9, Polysorbate 80, Sodium lauryl sulfate, and Sorbitan monopalmitate.
Ointment bases are a semi-solid vehicle for ointments. Examples of ointment bases are-
- Hydrophilic ointment
- Polyethylene glycol ointment
- Hydrophilic petrolatum
- White ointment
- Yellow ointment
- Rose water ointment
Suppositories bases, a vehicle for suppositories, are the most common dosage forms used to induce a local therapeutic effect. Suppositories are intended for insertion in body cavities like the rectum and vagina. Examples of suppository bases are Cocoa butter, Polyethylene glycols (mixtures), and PEG 3350.
These are the agents that are used to elevate (increase) the viscosity by reducing the sedimentation rate of particles in a vehicle in which they are insoluble. These can be incorporated for any route like oral, parenteral, ophthalmic, or topical. Examples of suspending agents are –
- Carbomer (e.g., Carbopol)
- Carboxymethylcellulose sodium
- Hydroxyethyl cellulose
- Hydroxypropyl cellulose
- Hydroxypropyl methylcellulose
The pharmaceutical water that is used in the production of parenterals and other formulations in which endotoxin content must be controlled is known as Water for injection (WFI). Water for injection (WFI) is also used for cleaning certain equipment used in the production of parenteral preparations. Water is highly used as an ingredient and solvent in several pharmaceutical formulations. Therefore, various countries including United States, Europe, and Japan define WFI as an excipient for various parenteral preparations in their individual Pharmacopoeia (USP, Ph.Eur., JP).
Coloring agents or colorants are pharmacologically inactive dyes or pigments that are used to impart color to liquid and solid (tablets and capsules) formulations. Caramel, Ferric oxide, and Regulatory approved colors are the coloring agents used in drugs.
Anti-adherents, also known as non-sticking agents, are used to prevent the adhesion of tablet products to dies and punches during production. Magnesium stearate is highly as used anti-adherent.
Stiffening agents are defined as agents that help to increase the hardness and thickness of various preparations such as ointments. These are viscosity-increasing agents used mainly for topical preparations. Widely used stiffening agents are –
- Cetyl alcohol
- Cetyl esters wax
- Microcrystalline wax
- Stearyl alcohol
- White wax
- Yellow wax
Chelating agents are the agents which form a chelate (water-soluble complex) when they react with metals. Chelate-forming agents are used as stabilizers in many liquid formulations for complexing heavy metals which further leads to instability. Examples of chelating agents are Edetic acid and Edetate disodium.
Aerosol propellants are the agents, which are responsible for the pressure developed in the aerosol containers which expel the product on the opening of the valve of containers. Examples of aerosol propellants are-
- Carbon dioxide
- Dichlorotetrafl uoroethane
Antioxidants are substances that prevent the deterioration of pharmaceutical preparations by resisting the oxidation of substances. Some of the common anti-oxidants are EDTA (Ethylenediaminetetraacetic acids), Ascorbic acid, Sod. Bisulfite, Sodium Metabisulfite, Sodium Formaldehyde sulfoxylate, and Ascorbic acid esters.